6.2C - Biological Carbon
Biological carbon can be stored as dead organic matter in soils, or returned to the atmosphere via biological composition over several years.
All living organisms contain carbon; the human body is about 18% carbon by weight. In plants, carbon dioxide and water are combined to form simple sugars, i.e. carbohydrates. In animals, carbon is synthesised into complex compounds, such as fats, proteins and nucleic acids.
On land, soils are the largest carbon stores. Here, biological carbon is stored in the form of dead organic matter. This matter can be stored for decades or even centuries before being broken down by soil microbes (biological decomposition) and then either taken up by plants or released into the atmosphere.
Soils store between 20% and 30% of global carbon. They sequester about twice the quantity of carbon as the atmosphere and three times that of terrestrial vegetation. The actual amount of carbon stored in some soil depends on:
- climate
- this dictates the rates of plant growth and decomposition; both increase with temperature and rainfall
- vegetation cover:
- this affects the supply of dead organic matter, being heaviest in tropical rainforests and least in tundra.
- soil type:
- clay protects carbon from decomposition, so clay-rich soils have a higher carbon content
- land use;
- cultivation and other forms of soil disturbance increase the rate of carbon loss